The invention relates to a device for cooling and granulating molten thermoplastic strands, which emerge from dies and which either drop along the falling line starting at the dies or are guided in an operating position through a coolant bath, freely stressed between guide rollers, arranged in said bath and then are fed into granulator, where a precooling device, which carries the strands emerging from the dies and takes them to the coolant bath, is arranged in the region between the dies and the coolant bath.
Such a device is known from the DE-PS 11 76 346. In this device between the dies and the coolant bath there are cooling rollers, into which the strands coming from the dies have to be threaded, in order to be guided thereafter through the coolant bath. In the coolant bath the strands travel through two pairs of deflecting rollers, which provide that the strands are guided reliably through the coolant bath. After leaving the coolant bath, the strands are then carried by a revolving conveyor belt, in order to then travel into the inlet of the granulator. In this device the strands must be threaded by hand, as customary with the use of such coolant baths, in the region between the dies and the granulator into the organs guiding and carrying the strands. This work is not only time-consuming, but it is also a dangerous operation, since the responsible personnel for monitoring such a device must reach into the hot strands. This is difficult, above all, when a larger number of strands, e.g. 50 strands and more, has been produced due to the width of the space that the strands occupy. At every startup of the device one has to wait until a qualitatively perfect material emerges from the dies, a state which, according to experience, takes some time following startup, during which the strands are guided first along their falling line next to the device into a waste container. If the operating personnel determines the quality of the emerging strands to be flawless, then a knife-like tool has to be slid by hand over the dies, in order to produce a break in the strands, after which then the newly emerging strands are guided into the coolant bath. The more strands issue from the dies, the more difficult this operation is, so that with this manual manipulation only one portion of the dies can be stripped off in the described manner, so that the strands emerging from the other dies continue to fall into the waste container.
In addition, a device for cooling molten thermoplastic strands emerging from dies is known from the DE-AS 22 30 187; in said device the strands are guided to a pair of conveyor belts, by means of which the strands and the conveyor belts are guided in a meandering guide through a coolant bath.
Furthermore, it is known from the DE-AS 12 95 176 to cool the strands emerging from the dies in a coolant bath, in which the strands are caught by a collecting pan, before the strands are fed into a granulator. The collecting pan is slid along a slideway, whereby the strands are supposed to catch apparently in an intercepting slot of the collecting pan. They are taken along by the displaced collecting pan and thus pulled out of the coolant until they get to the granulator. To thread the granulator, the strands are cut off upstream of the collecting pan, so that a quantity of plastic, guided beforehand into the collecting pan, remains first in said pan and has to be removed for the next working step.
The invention is based on the problem of automating the threading operation of the strands in such a manner that it is ensured that the entire number of delivered strands can always enter with their forward ends simultaneously into the granulator.
This problem is solved according to the invention by disposing behind the precooling device for a clamping grasp of the forward ends of the strands a gripping device, which guides the forward ends of the strands--after the strands have been jointly separated by means of a separator moveable through the falling line of the strands--along a guiding rail running through the region of the coolant bath as far as the granulator.
The gripping device is always offered in this manner the forward ends of the strands that are produced after separating the strands by means of the moveable separator, so that the forward ends of the strands are subsequently threaded by means of the gripping device moved along the guiding rail virtually without mutually offsetting the forward ends of the strands in the longitudinal direction. Thus, the gripping device guided along the guiding rail can reliably grasp the forward ends of the strands, which are cut by the separator and from which every opportunity has been taken in the gripping device to stick to the neighboring strands. Thus, the forward ends of a flawless material reach in essence simultaneously the granulator, so that said granulator can immediately cut the strands into pellets of high value quality. In contrast, when a larger number of strands are threaded by hand, the forward ends of the strands always form lumps, after which the strands continue to flow in more or less stuck together for some time. The granulator cannot cut perfect pellets from this mass of delivered material, since pellets can be cut only from clean single strands. The gripping device is provided expediently with opposed clamping rollers to grip the forward ends of the strands, since said gripping device can also exert a feeding motion on the strands.
The guiding rail and thus the gripping device can be guided as far as immediately in front of the granulator, from where then the strands can travel directly into the inlet of the granulator. However, it is also possible to let the guiding rail terminate in an end position, in which the rollers are set into motion in the sense of a feed motion, until the strands are grasped by the granulator. This kind of feed of the strands to the granulator has the advantage that it is not necessary to bring the gripping device close up to the granulator, a feature that in the case of irregularities in the position of the individual strands in the gripping device could lead to single strands not being grasped at all by the granulator. If, however, the gripping device with its rollers can feed the strands into the granulator from a specific distance up stream of the granulator, and in particular due to the corresponding rotation of the rollers, then this method of operating the gripping device guarantees that in the final analysis all of the strands can also be grasped reliably by the granulator.
With precooling the goal of designing the precooling device as a discharge channel, which is fed with the coolant and which is arranged underneath the dies to catch the strands, can be achieved. This discharge channel is a simple component, which can be accessed from the top and on which the strands falling from the dies are automatically oriented subject to the influence of the coolant stream flowing over the discharge channel. In the region of the discharge channel the strands are adequately cooled, in order to be then grasped as forward ends by the gripping device without any significant deformation.
The discharge channel can be combined in a manner that makes sense from a technological point of view with the separator, namely by rendering the discharge channel moveable along a carriage guide and by assigning to it the separator, which while moving the front side of the discharge channel grasps and separates the strands through the falling line of the strands into the operating position.
With such a moveable discharge channel it can be achieved that in its external operating position said discharge channel lets the strands in the falling line fall into a waste container, a process that is necessary, as explained above, during startup of the system, whereupon then when the strands of flawless material emerge, the discharge channel can be slid with its front side through the falling line of the strands, whereby the separator allocated to it grasps and jointly separates the strands. It should also be pointed out that such a relatively short discharge channel with a separator interacting with another discharge channel is known from the DE-PS 32 05 052.
The guiding rail runs expediently through the coolant bath and around the guide rollers, so that if the gripping device runs in an analogous manner, the strands grasped by said gripping device are guided through the coolant bath and thus also around the guide rollers, thus completing the automatic threading operation. At the same time, however, the guiding rail must run through corresponding curves in the region of the inlet and the outlet to the coolant bath. However, it is also possible to design the guiding rail in essence linearly. To this end, the guide rollers can be moved in such a manner that they can be lifted out of the coolant bath and lowered into said bath, and that the guiding rail runs between coolant bath and the elevated position of the guide rollers, whereby upon reaching the end position the guide rollers are moved into the lowered position by means of the gripping device.
In the design of the guiding rail, the strands are first moved freely suspended over the coolant bath, whereupon the previously elevated guide rollers, below which the guiding rail runs, are lowered, whereby they dip the strands into the coolant bath. Then the strands continue to run around the guide rollers through the coolant bath.